Power semiconductor module

ABSTRACT

A power semiconductor module has a circuit assembly body, which includes a metal base, a ceramic substrate, and a power semiconductor chip, and is combined with a package having terminals formed integrally. The ceramic substrate of the module has a structure such that an upper circuit plate and a lower plate are joined to both sides of a ceramic plate, respectively, and the metal base and the ceramic substrate are fixed to one another using solder, thereby improving reliability and lengthening a life of a power semiconductor module by optimizing a ceramic substrate and a metal base thereof, the dimensions thereof, and material and method used for a join formed between the ceramic substrate and metal base.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Pat. No. 6,690,087filed Dec. 28, 2001, now issued and incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power semiconductor module that maybe applied to a converter or inverter of a power switching device, forexample.

2. Description of the Related Art

First, the assembled structure of a conventional example of such a powersemiconductor module (in which six elements form a structure for threecircuits), will now be described using FIGS. 9 a and 9 b. In thefigures, a metal base (copper base plate) 1 is provided for radiatingheat, which is a heat sink, an outer case 2 is provided having terminalsformed integrally, and a main circuit terminal 3 is provided in thisouter enclosing case 2. A control terminal 4, a power circuit block 5, acontrol circuit block 6, and bonding wires 7 including internal wiringare also provided.

FIGS. 9 a and 9 b illustrate the power circuit block 5 having six ofeach of a power semiconductor chip 9, such as an IGBT, and afree-wheeling diode 10, mounted on a ceramic substrate 8 that is in theshape of a rectangular oblong. Further, as shown by the schematic viewof FIG. 4, the ceramic substrate 8 is a direct copper bonding substrateincluding an upper circuit plate 8 b, and a lower plate 8 c, which areplated with copper foil and are directly joined to the upper surface andlower surface, respectively, of a ceramic plate 8 a made from alumina,aluminum nitride or silicon nitride. A semiconductor pattern is formedon the upper circuit plate 8 b. The semiconductor chips 9 are mounted onthe upper circuit plate 8 b via a solder join layer 11 including solder.Referring to FIGS. 9 a and 9 b, the control circuit 6 is made such thatdrive ICs 6 b, which are for driving power circuit elements, and circuitcomponents associated with these drive ICs, are mounted on a U-shapedprinted substrate 6 a surrounding the power circuit block 5.

In addition, the power circuit block 5 and the control circuit block 6are placed side by side on a metal base 1. Thereupon, the lower plate 8c of the ceramic substrate 8, on the power circuit block 5, and themetal base 1 are joined using Sn—Pb solder (this solder is representedby “12”) to conduct heat generated by the power semiconductor chips 9 tothe metal base 1 via the ceramic substrate 8. Further, the printedsubstrate 6 a of the control circuit block 6 is affixed to the metalbase 1 using an adhesive.

The outer case 2 having terminals integrally formed therein is joined tothe metal base 1 using an adhesive. Specifically, the outer case 2 isplaced over the power circuit block 5 and the control circuit block 6 ismounted on the metal base 1 so as to enclose the power circuit block 5and the control circuit block 6. The inner lead portions 3 a of the maincircuit terminals 3 are soldered to the printed pattern of the ceramicsubstrate 8, and control terminals 4 become internal wiring as a resultof bonding wires 7 to the printed pattern of the printed substrate 6.Further, after a gel-like filler material (silicon gel, for example),has been injected into the outer enclosing case 2 to seal off each ofthe circuit blocks using resin, the outer enclosing case is covered witha lid (not shown) to thus complete the power semiconductor module.

Power semiconductor modules like those mentioned above are extensivelyemployed in various fields, from low-capacity devices for everyday useto high-capacity devices which are used industrially or in vehicles, forexample. In such cases, low-capacity modules for everyday-use, for whichthe degree of reliability required is not particularly high, and inwhich the amount of heat generated by power semiconductor elements islimited, can be designed and manufactured with barely any restrictionsbeing placed on the material and size of each of the components. On theother hand, where high-capacity modules are concerned, which aretypically used in power circuits of vehicle drive devices, there issometimes an increased amount of heat generated per unit area of powerchips in the high-capacity modules. Because the size of such power chipsis smaller and more compact, there is a demand for high reliability andlong life in high-capacity modules compared to the low-capacity modulesin products manufactured for everyday use.

For example, a heat cycle test to which manufactured power modules aresubjected to include conditions for one heat cycle: −40° C. (60minutes), followed by room temperature (30 minutes), followed by 125° C.(60 minutes), and followed by room temperature (30 minutes). Incontrast, ordinary general-purpose products are subjected to a test ofaround 100 cycles. Accordingly, the reliability required for productsmanufactured for vehicle drive devices is such that such products mustsufficiently withstand a heat cycle test of 3000 cycles.

Satisfying these requirements of high reliability and long lifenaturally means improving the reliability of the power semiconductorchips themselves. In addition, a pressing problem regarding these powersemiconductor chips includes ensuring a durability that can sufficientlywithstand high levels of heat radiation, and the severe heat cyclesaccompanying the motion of the vehicle, at the same time as ensuring therequired electrical insulation resistance.

In a structure in which a ceramic substrate 8 mounted on a metal base 1is joined thereto with solder, as shown in FIG. 4, the problems setforth above are accompanied by the problems noted hereinbelow.

(1) Thermal stress attributable to a difference in the thermal expansionof the ceramic substrate/metal base: specifically, while a rate ofthermal expansion of the ceramic substrate is 7 ppm/K for alumina, 4.5ppm/K for aluminum nitride, and 3 ppm/K for silicon nitride, a rate ofthermal expansion of the metal base (copper base) is 16.5 ppm/K, whichprovides for a large difference between the thermal expansion rates ofthe ceramic substrate and the metal base. Meanwhile, the yield strengthof the solder (Sn—Pb solder), which forms the join between the metalbase and the ceramic substrate, is low at around 35 to 40 MPa, and, whenthe stress at the solder join portion repeatedly increases withsuccessive heat cycles on account of the difference between the thermalexpansion rates of the metal base and the ceramic substrate, this soldereventually burns out.

In addition, with regard to a life of the solder until burn-out, afterperforming various testing on the life of the solder in addition to acorresponding analysis, it has been established that there was atendency for the life of the solder to be heavily dependent on theconditions below. In other words, assuming that the ceramic material ofthe ceramic substrate soldered onto the metal base is the same: (a) anincrease in the plate thickness of the ceramic substrate entails anincrease in the strain generated in the solder join portion, and thelife before burn-out becomes short (dependence on the thickness of thesubstrate); (b) an increase in the surface area of the ceramic substrateentails an increase in the strain generated in the solder join portion,and the life before burn-out becomes short (dependence on the surfacearea of the substrate); (c) an increase in the ratio of the length andbreadth of the ceramic substrate entails an increase in the straingenerated in the solder join portion, and the life before burn-outbecomes short (dependence on the shape of the substrate); and (d) adecrease in the thickness of the solder constituting the solder joinlayer entails an increase in the strain generated, and the life beforeburn-out becomes short (dependence on the thickness of the solder).

(2) Thermal conductivity between the ceramic substrate/metal base: Theheat generated by the power semiconductor chip is thermally conductedvia the ceramic substrate to the metal base, which functions as a heatsink, and is radiated from the metal base to the outside. Consequently,it is necessary to keep the resistance to thermal conduction as low aspossible between the ceramic substrate and the metal base.

Accordingly, (a) while a thermal conduction rate of the ceramicsubstrate is 20 W/mK for alumina, 180 to 200 W/mK for aluminum nitride,70 to 100 W/mK for silicon nitride, and the thermal conduction rate ofthe copper base is 398 W/mK, the thermal conduction rate of the solder(Sn—Pb solder), which forms a join between the metal base and theceramic substrate, is 40 to 50 W/mK. As a result, a thermal resistanceof the solder join portion has a major influence on the thermalconduction in the thermal conduction path between the powersemiconductor chip and the metal base. (b) Further, upon soldering theceramic substrate onto the metal base, when voids (air bubbles) aregenerated in the solder join layer, these voids provide for a thermalresistance and therefore obstruct the radiation of heat. Consequently,an increase in the thickness of the solder layer of the solder joinlayer entails lower heat radiation, which has an adverse effect on thereliability and durability of the module.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a powersemiconductor module in which the material of the ceramic substrate andthe metal base, the dimensions thereof, the material for joining theceramic substrate and the metal base, and the joining method, areoptimized on the basis of an analysis of the results of each of theabove-mentioned tests, and in which module the thermal strain, which isgenerated between the ceramic substrate and the metal base withsuccessive heat cycles, is alleviated and thermal conductivity isimproved, thus it becomes possible to ensure the high degree ofreliability and long life that are required for use in vehicles.

The present invention provides a power semiconductor module, including:a metal base including a heat sink; a circuit assembly body including anupper circuit plate, a lower plate, a ceramic plate including an uppersurface and a lower surface; a ceramic substrate placed on the metalbase including the upper circuit plate and the lower plate, wherein theupper circuit plate and the lower plate are joined to the upper surfaceand lower surface of the ceramic plate, respectively, and asemiconductor chip, wherein the semiconductor chip is placed on theceramic substrate; an outer case including terminals formed integrallytherein; a join formed between the metal base and the lower plate of theceramic substrate; and a power semiconductor module assembled by forminga join using solder between the metal base and the ceramic substrate.

The solder includes a melting point of 183 to 250° C. and the thicknessof the solder layer is at least 0.1 mm and no more than 0.3 mm. Theupper circuit plate and the lower plate are made of metal. The metalbase includes copper or a copper alloy with a thermal conduction rate ofat least 250 W/mK. A plate thickness of the metal base includes betweenabout 3.9 mm to 6 mm. A thickness of the ceramic plate of the ceramicsubstrate includes between about 0.1 mm to 0.65 mm, a thickness of theupper circuit plate and of the lower plate includes between about 0.1 mmto 0.5 mm, an overall size of the ceramic substrate includes 50 mm×50mm, and a ratio of a length and a breadth of the ceramic substrateincluding 1:1 to 1:1.2.

The ceramic plate of the ceramic substrate is aluminum nitride, and theupper circuit plate and the lower plate are aluminum. The ceramic plateof the ceramic substrate is silicon nitride, and the upper circuit plateand the lower plate are copper. The power semiconductor module comprisesthe ceramic plate of the ceramic substrate is alumina, and the uppercircuit plate and the lower plate are copper and placing at least oneceramic substrate on the metal base.

The present invention is also achieved by a power semiconductor module,including: a metal base including a heat sink; a semiconductor chip; aceramic substrate; a circuit assembly body including the ceramicsubstrate, wherein the semiconductor chip is placed on the ceramicsubstrate which is then placed on the metal base; and an outer casehaving terminals formed integrally therein, wherein the ceramicsubstrate is integrally formed on the metal base by successivelyspray-forming a ceramic layer of the ceramic substrate, and a copperlayer of the upper circuit plate, on the surface of the metal base. Theceramic layer is any one of alumina, aluminum nitride, and siliconnitride. Aluminum is spray-formed on the surface of the copper layer toform an upper circuit plate bonding pad.

The present invention is also achieved by a power semiconductor module,including: a metal base including a heat sink; a semiconductor chip; aceramic substrate; a circuit assembly body including a ceramic plate, anupper circuit plate, and the ceramic substrate, wherein thesemiconductor chip is placed on the ceramic substrate which is thenplaced on the metal base; and an outer case having terminals formedintegrally therein, wherein a casting method integrally molds the metalbase on the underside of the ceramic plate of the ceramic substrate,which has no lower plate. and in which the upper circuit plate is joinedto the ceramic plate.

The present invention also provides a power semiconductor module,including: a metal base including a heat sink; a semiconductor chip; aceramic substrate; a circuit assembly body including the ceramicsubstrate, wherein the semiconductor chip is placed on the ceramicsubstrate which is then placed on the metal base; and an outer casehaving terminals formed integrally therein, wherein an upper circuitplate and the metal base are formed directly by a formation of a layerof molten metal on an upper face and a lower face of the ceramic plate,respectively.

The present invention also provides a power semiconductor module,including: a metal base including a heat sink; a semiconductor chip; aceramic substrate; a circuit assembly body including a circuit assemblybody including an upper circuit plate, a ceramic plate, and the ceramicsubstrate, wherein the semiconductor chip is placed on the ceramicsubstrate which is then placed on the metal base; and an outer casehaving terminals formed integrally therein, wherein the ceramicsubstrate has no lower plate and in the ceramic substrate, a silver waxmaterial brazes the upper circuit plate joined to the ceramic plate andthe metal base.

These together with other objects and advantages, which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objective and advantage of the present invention will becomemore apparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 a is a schematic side view showing an assembly body including ametal base/a ceramic substrate/a power semiconductor chip, relating to afirst embodiment of the present invention;

FIG. 1 b is a schematic view showing a planar view of the assembly bodyof FIG. 1 a;

FIG. 2 is a figure showing an assembly body including a metal base/aceramic substrate/a power semiconductor chip, according to a second anda third embodiment of the present invention;

FIG. 3 is a figure showing an assembly body including a metal base/aceramic substrate/a power semiconductor chip, according to a fourthembodiment of the present invention;

FIG. 4 is a figure showing a conventional power semiconductor modulehaving an assembly body including a metal base/a ceramic substrate/apower semiconductor chip;

FIG. 5 a is a figure showing a planar view of a manufactured powersemiconductor module having one ceramic substrate, according to thepresent invention;

FIG. 5 b is a figure showing a planar view of the manufactured powersemiconductor module having one ceramic substrate with an outerenclosing case removed, according to the present invention;

FIG. 6 a is a figure showing an exploded perspective of a manufacturedpower semiconductor module having two ceramic substrates, according tothe present invention;

FIG. 6 b is a figure showing a side view of the manufactured powersemiconductor module having two ceramic substrates with the outerenclosing case removed, according to the present invention;

FIG. 7 a is a figure showing a planar view of a circuit assembly body inthe power semiconductor module of FIG. 6;

FIG. 7 b is a figure showing a side view of a circuit assembly body inthe power semiconductor module of FIG. 6;

FIG. 8 a is a figure showing a planar view of a manufactured powersemiconductor module having six ceramic substrates, according to thepresent invention;

FIG. 8 b is a figure showing a cross-sectional view seen from a side ofa manufactured power semiconductor module having six ceramic substrates,according to the present invention;

FIG. 9 a is a figure showing an exploded view of a conventional powersemiconductor module; and

FIG. 9 b is a perspective view of an outside of the conventional powersemiconductor module of FIG. 9 a in an assembled condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will be now made in detail to the present exemplaryembodiments of the present invention, examples of which are illustratedin the accompanying drawings. Further, a description has been omittedfor members corresponding to FIGS. 4 through 9 and to which the samereference numerals have been assigned in each of the embodiments.

First Embodiment

FIG. 1 a is a schematic side view showing an assembly body including ametal base/a ceramic substrate/a power semiconductor chip, relating to afirst embodiment of the present invention. FIG. 1 b is a schematic viewshowing a planar view of the assembly body of FIG. 1 a. A structure of aceramic substrate 8 is such that an upper circuit plate 8 b and a lowerplate 8 c are joined to the upper surface and lower surface of a ceramicplate 8 a, respectively; and a join is formed using solder between themetal base 1 and the lower plate 8 c of the ceramic substrate 8.

Here, the material of the metal base 1 is copper or a copper alloy,which has a thermal conduction rate equal to or greater than 250 W/mK. Aplate thickness t1 of this metal base is 3.9 mm to 6 mm. A thickness t2of the ceramic plate 8 a of the ceramic substrate 8 is 0.1 mm to 0.65mm, and the thickness of the upper circuit plate 8 b and of the lowerplate 8 c is 0.1 mm to 0.5 mm. An overall size (a×b) of the ceramicsubstrate 8 is set to be at most 50 mm×50 mm, and the ratio of thelength and breadth thereof is set within a range of 1:1 to 1:1.2.Further, the metal base 1 and the ceramic substrate 8 are assembled byforming a join therebetween using solder, where the solder employed isSn—Pb solder having a melting point of 183 to 250° C. and the thicknessof the solder join layer 12 is 0.1 to 0.3 mm.

Further, combining materials such as those detailed hereinafter mayinclude the ceramic substrate 8. Specifically, (1) the material of theceramic plate 8 a may be aluminum nitride, and the material of the uppercircuit plate 8 b and the lower plate 8 c, which are joined to the upperside and the lower side of the ceramic plate 8 a, may be aluminum; (2)the material of the ceramic plate 8 a may be silicon nitride, and thematerial of the upper circuit plate 8 b and the lower plate 8 c may becopper; and (3) the material of the ceramic plate 8 a may be alumina,and the material of the upper circuit plate 8 b and the lower plate 8 cmay be copper.

Furthermore, a power semiconductor module, as exemplified in FIGS. 5 athrough 8 b, includes: the material of the metal base 1 and the ceramicsubstrate 8, the size thereof, and the material and thickness of thesolder, as described above. By way of example, FIG. 5 a and FIG. 5 bshow examples of manufactured products in which a power circuit block 5is includes one ceramic substrate 8 mounted on a metal base 1, where themetal base 1 is made from a copper alloy and has a plate thickness of 4mm, and an overall size of length 86 mm and breadth 107 mm. Further, theceramic plate 8 a of the ceramic substrate 8 is an alumina plate with aplate thickness of 0.32 mm, and an overall size of length 51 mm andbreadth 50 mm, where the length to breadth ratio is 1:1.02. A join isformed using Sn—Pb solder between the metal base 1 and the ceramicsubstrate 8, where a thickness of the solder join layer is 0.15 mm.

In addition, in the example of a manufactured product shown in FIGS. 6 aand 6 b, and FIGS. 7 a and 7 b, the power circuit block 5 is dividedinto two ceramic substrates 8, which are placed side by side on themetal base 1, where the metal base 1 is made from a copper alloy havinga plate thickness of 4 mm and an overall size of length 68 mm andbreadth 98 mm. In turn, ceramic plates of the ceramic substrates 8 maybe alumina plates with a thickness of 0.25 mm, and an overall size of aceramic plate of the ceramic substrate 8 positioned on a left of thepower circuit block 5 may include: a length of 38 mm and a width of 33mm. An overall size of a ceramic plate of ceramic substrate 8 positionedon a left of the power circuit block 5 may include: a length of 38 mm, awidth of 39 mm, where a length to breadth ratio of the left and rightplates may be 1:1.15 and 1:1.02, respectively. Further, a join is formedusing Sn—Pb solder between the metal base 1 and each of the ceramicsubstrates 8, respectively, positioned on the left and right of thepower circuit block 5, where the thickness of the solder join layer maybe 0.15 mm.

Further, in the example of a manufactured product shown in FIGS. 8 a and8 b, the power circuit block 5 is divided into six ceramic substrates 8,which are placed side by side on the metal base 1, where the metal base1 may be made from a copper alloy with a plate thickness of 4 mm, andwith an overall size of length 122 mm and breadth 162 mm. Further, aceramic plate of each of these ceramic substrates 8 is an alumina platewith a thickness of 0.32 mm, and with an overall size of length 38 mm,and width 41.9 mm, where the length to breadth ratio of these plates is1:1.1. A join is formed using Sn—Pb solders between the metal base 1 andeach of the ceramic substrates 8, where the thickness of the solder joinlayer is 0.15 mm.

Second Embodiment

A second embodiment of the present invention will be describedhereinbelow referring to FIG. 2. In this embodiment, without usingsolder in a formation of a join between the metal base 1 and the ceramicsubstrate 8, the ceramic substrate 8 is integrally molded directly ontothe metal base 1 using the method described below.

Specifically, by spray-forming a ceramic material such as alumina,aluminum nitride or silicon nitride onto the surface of the metal base 1(one face thereof) by means of a plasma spray-forming method, a ceramiclayer, which corresponds to the ceramic plate 8 a and having thicknessof 0.1 to 0.65 mm, is formed covering and adhering to the surface of themetal base 1. Next, copper is spray-formed, using the plasmaspray-forming method, on this ceramic layer, where a copper layer, whichcorresponds to the upper circuit plate 8 b and has a thickness of 0.1 to0.5 mm, is formed as a lamination layer thereon. Further, copper,aluminum or molybdenum, or a combination of these materials, may beemployed for the metal base 1.

Further, the above-mentioned ceramic layer and copper layer, which areformed on the metal base using the plasma spray-forming method,correspond to the ceramic plate 8 a of the ceramic substrate 8, and theupper circuit plate 8 b, respectively. A power semiconductor chip 9, forexample, is mounted using solder on this upper circuit plate 8 b. It isalso possible to form a bonding pad by spray-forming an alumina layer onthe copper layer.

By means of such a constitution, since there is no solder layerinterposed between the metal base 1 and the ceramic substrate 8, thermalresistance is diminished by an amount representing this solder or thelike, and the heat radiation is thereby improved. In addition, since nosolder join is employed, the soldering process and the solder materialare omitted, and the problems of an increase in the thermal resistanceattributable to voids in the solder layer, and of the solder join layerburning out as a result of thermal strain therein, are also resolved.

The ceramic layer, which is spray-formed on the metal base 1, is notrestricted to one particular kind of ceramic material. Alumina, forexample, may be spray-formed on the metal base 1, and, subsequently,aluminum nitride, silicon nitride or another insulating material may bespray-formed on the resulting laminated metal base 1 to form laminationlayers.

Third Embodiment

Next, a third embodiment of the present invention, will be describedthrough reference to FIG. 2. In this embodiment, a ceramic substrate 8,which has no lower plate and in which an upper circuit plate 8 b is madeusing copper foil or the like, is joined to the surface of a ceramicplate 8 a, and has a metal base 1 integrally molded with the undersideof the ceramic plate 8 a using a casting method. The metal base 1 ismade from aluminum (melting point: 500 to 700° C.), which has a lowermelting point than copper (melting point: 1085° C.).

In this casting method, the underside of the ceramic plate 8 a of theceramic substrate 8 is set in a mold. The mold is full of molten metalthat is to be the material of the metal base, such that the underside ofthis ceramic plate is soaked with this molten metal, and the metal base1 is cast in these conditions alone. A metal base 1 is thus formed thatis integrally connected to the ceramic substrate 8.

In this embodiment, similarly to the second embodiment described above,because no solder layer is interposed between the metal base 1 and theceramic substrate 8, the problems of an increase in the thermalresistance, and of the solder join layer burning out as a result ofthermal strain therein, are also resolved.

As an applied example of this embodiment in which a casting method isemployed, it is possible to use molten metal to integrally form an uppercircuit plate 8 b and a metal base 1 on the upper face and lower face ofthe ceramic plate 8 a, respectively.

Fourth Embodiment

Next, a fourth embodiment of the present invention, will be describedusing FIG. 3. In this embodiment, a metal base 1 and a ceramic substrate8, which has no lower plate and in which an upper circuit plate 8 b hasalready been joined to a ceramic plate 8 a, are combined using silverwax 13. This fixing operation using wax involves inserting silver wax inthe form of a sheet or a cream between the metal base 1 and the ceramicplate 8 a of the ceramic substrate 8, and then introducing the resultingassembly into a reflow furnace to cause this fixing using wax.

In this constitution, the thermal conduction rate of the silver waxmaterial is equal to or greater than 300 W/mK, which is at least sixtimes higher than the thermal conduction rate of 40 to 50 W/mK of Sn—Pbsolder. Therefore, in addition to being able to obtain a high degree ofheat dissipation in comparison with the conventional structure shown inFIG. 4, a yield strength is high in comparison with that of solder, andreliability with respect to burn-out is also improved. Moreover,similarly to the first embodiment, the second through fourth embodimentsdescribed above may be applied to each of the different types ofmanufactured power semiconductor module shown in FIGS. 5 through 8.

As described above, the following effects are afforded by the presentinvention:

(1) The following are optimally set: the material of the ceramicsubstrate, which has a power semiconductor chip mounted thereon, and ofthe metal base for radiating heat, which is a heat sink; the dimensionsthereof; the solder material, which joins the ceramic substrate and themetal base; and the thickness of the solder layer. Such a constitutionmakes it possible to obtain a high degree of heat radiation and toreduce thermal strain, which is generated between the ceramicsubstrate/metal base with successive heat cycles during electrificationof the power semiconductor module. Further, the constitution ensures therequired insulation resistance. It is thus possible to obtain a powersemiconductor module that satisfies the conditions of high reliabilityand durability that are required for use, for example, in vehicles.

(2) Further, in the embodiments in which no solder is employed in thejoining together of the metal base and the ceramic substrate, and thelower plate of the ceramic substrate is omitted such that a direct joinis made with the metal base, because there is nothing interposed betweenthe metal base and the ceramic substrate, such as solder, the thermalresistance is reduced by an amount representing this solder or the like,and the heat radiation is thereby improved. In addition, because nosolder join is employed, the soldering process and the solder materialare omitted. Accordingly, the problems of an increase in the thermalresistance attributable to voids in the solder layer and of the solderjoin layer burning out as a result of thermal strain therein are alsoresolved.

(3) Moreover, in the embodiments in which a metal base and a ceramicsubstrate are fixed together using silver wax material, it is possibleto afford the module excellent heat radiation in comparison with amodule which is joined together using solder, as well as high durabilitywith respect to burn-out.

1. A power semiconductor module, comprising: a metal base comprising aheat sink; a semiconductor chip; a ceramic substrate; a circuit assemblybody comprising a circuit assembly body comprising an upper circuitplate, a ceramic plate, and the ceramic substrate, wherein thesemiconductor chip is placed on the ceramic substrate which is thenplaced on the metal base; and an outer case having terminals formedintegrally therein, wherein the ceramic substrate has no lower plate andin the ceramic substrate, a silver wax material brazes the upper circuitplate joined to the ceramic plate and the metal base.
 2. The powersemiconductor module according to claim 1, wherein the powersemiconductor module comprises placing at least one ceramic substrate onthe metal base.